Metal cans are roughly classified into two-piece cans and three-piece cans. The two-piece can refers to a can composed of two parts of a can body with an integral can bottom and a lid. The three-piece can refers to a can composed of three parts of a can barrel, a top lid, and a bottom lid. The can body of the two-piece can has beautiful appearance because no seam portion (welded portion) is included. However, a high strain level is usually required. The can barrel of the three-piece can is inferior in appearance to the two-piece can because a seam portion is included. However, the strain level is usually low. Consequently, two-piece cans are used for small-capacity, high-quality goods in many cases and three-piece cans are used for large-capacity, low-price goods in many cases, as a trend in the market.
In general, expensive, thick aluminum sheets are used as metal materials for can bodies for two-piece cans, e.g., aerosol cans, wherein the strain in drawing is high and a degree of elongation in a can height direction is large (hereafter may be referred to as high strain), among such two-piece cans. Inexpensive, thin steel sheet materials, e.g., tinplate and tin-free steel, are hardly used. The reason therefor is that the strain of the aerosol two-piece can is very high, and it is difficult to apply high degree of working, e.g., drawing or DI (draw and ironing), to the steel sheet, whereas an impact forming method can be applied to soft metal materials, e.g., aluminum.
Under such circumstances, if the can body of the above-described high-strain two-piece can is allowed to be produced by using an inexpensive, thin, high-strength steel sheet material, e.g., tinplate or tin-free steel, the industrial significance is very high.
In a previously known technology, a common low-strain two-piece can is produced from a resin-laminated steel sheet (may be referred to as a laminated steel sheet) serving as a raw material by a drawing method or a DI method.
For example, Japanese Examined Patent Application Publication No. 7-106394, Japanese Patent No. 2526725, Japanese Unexamined Patent Application Publication No. 2004- and Japanese Examined Patent Application Publication Nos. 59-35344 and 61-22626 disclose the drawing method or the DI method of resin-coating metal sheets. However, in every technology, targets are beverage cans, food cans, and the like, and are low-strain can bodies which do not require such a high strain level applied to two-piece aerosol cans.
Japanese Examined Patent Application Publication No. 7-106394 discloses a method in which a can body is formed by performing draw and ironing of a laminated steel sheet with a drawing die having a specific shape. Low-strain can bodies are formed by this method with no problem. However, if the method is applied to high-strain cans, e.g., aerosol cans, cracking, peeling, or the like occurs in a film during working.
Japanese Patent No. 2526725 and Japanese Unexamined Patent Application Publication No. 2004-148324 disclose that a heat treatment is performed in a working stage or a final stage for the purpose of preventing peeling of a resin layer and ensuring a barrier function after working. That is, in Japanese Patent No. 2526725, an orientational thermoplastic resin is used and a heat treatment for relaxing the internal stress and facilitating orientational crystallization is proposed. This heat treatment method is a technique which is generally used for beverage cans and the like at present. According to the description of Japanese Patent No. 2526725, it is desirable that the heat treatment is performed in the state of a cup subjected to redrawing at (melting point−5° C.) or lower at which an increase in crystallinity of the coating resin is facilitated adequately. However, it is clear that merely low-strain products are targeted, insofar as the description of the examples is concerned.
In the examples disclosed in Japanese Unexamined Patent Application Publication No. 2004-148324, a resin composed of a compound of a saturated polyester and an ionomer is disposed as a coating layer and working by DI is performed. In the working method of Japanese Unexamined Patent Application Publication No. 2004-148324, a heat treatment is performed following the drawing, and subsequently, working by DI, necking, and flanging are performed. However, it is also clear that merely low-strain products are targeted, insofar as the description of the examples is concerned.
Furthermore, in the methods described in Japanese Examined Patent Application Publication Nos. 59-35344 and 61-22626, after a can is formed, the resulting can is heat-treated mainly at a temperature higher than or equal to the melting point of a resin so as to relax the internal stress. However, the strain of the resulting can body is low insofar as the descriptions of the specification and the examples are concerned.
That is, at present, there is no method for manufacturing a can body of a high-strain two-piece can, e.g., an aerosol can, by using a laminated steel sheet.
We attempted to produce high-strain two-piece cans, wherein a circular laminated steel sheet was used and formed into the shape of a hollow cylinder with a bottom and, thereafter, the diameter of a portion in the vicinity of the opening portion was reduced. As a result, peeling and breakage of the resin layer occurred. These were problems specific to forming with high strain level. We believed that a heat treatment would be effective for overcoming the problem. However, previously known heat treatments after and before the forming were unsatisfactory, and we were not able to avoid peeling of the resin layer in a high-strain region. Consequently, the resin layer peeling problem was not able to be solved even when the prior art was applied. Furthermore, a problem also occurred in that the workability of the resin layer deteriorated in steps downstream from the heat treatment step.
It could therefore be advantageous to provide a can body for a laminated steel sheet two-piece can and a method for manufacturing the same, wherein the can body has a high-strain comparable to that of a two-piece can for aerosol, and peeling and breakage of a resin layer do not occur.